Two-chain noncontinuous complete partial order

ID: two-chain-noncontinuous-complete-partial-order

Take two increasing countable chains sharing a least element and a greatest element , with interiors and and no comparisons between branches. Each entire interior chain has as its least upper bound. A directed set meeting both interiors must contain . Otherwise it lies within one branch, where it has either a greatest member or least upper bound . This proves that the partially ordered set is a directed-complete partial order.
The only element way-below is . Indeed, an element way-below must be below , but every positive fails the way-below relation when tested against : its least upper bound is , yet it has no member above . Nonempty directed sets always contain an element above , so . Thus the elements way-below have least upper bound , and this is not a continuous poset.

New to topics? Read the docs here!